Use Firefox as default browser if present and drop iceweasel config

The "Iceweasel Age"[1] has ended and we can use Firefox again and
therefore use firefox as x-www-browser.

Unfortunately overriding the default homepage via
/etc/iceweasel/profile/prefs.js does not work anymore therefor dropping
the iceweasel configuration. (See: grml/grml#18)

[1] https://lwn.net/Articles/676799/

Closes grml/grml#15